But Duo is limited to video chat, while FB Messenger and WhatsApp are more comprehensive messaging apps. Video chatting isn't as universal as messaging. It'll be more interesting to see how Allo does in terms of downloads.
Also, FB almost forces Messenger on its users. It's possible to get by without it, since it's still possible to send FB messages on the browser site, but you have to trick it by using the Request Desktop Site option first. If you use the FB app, though, it's basically required.